Take a fairly trivial example (which has obviously been simplified to the point of absurdity):

import attr

@attr.s
class Test():
    attr = attr.ib(default=None)

test = Test()
other = Test()
test.attr = other
other.attr = test

print(repr(test))

This will result in infinite recursion. This can be worked around by setting repr=False in the attr.s decorator.

I've sat on this bug for a week trying to come up with a good solution, and I can't think of anything good. I'm starting to think the best solution is to add a warning to the documentation of this behavior and calling it good.